1. Field of the Invention
The present invention relates to a drive transmission changer for a dog clutch of a vehicle having a drive motor built therein.
2. Description of the Prior Art
An automotive vehicle having a two-wheel drive (2WD) mode, in which the drive output from the automotive power plant is transmitted only to rear or front wheels, and a four-wheel drive (4WD) mode, in which the drive output from the same automotive power plant is transmitted to the rear and front wheels, is known, in which a drive transmission changer is employed for selectively setting the automotive vehicle in one of the two-wheel drive modes and the four-wheel drive mode. The Japanese Laid-open Patent Publication No. 2004-187378, published Jul. 2, 2004, discloses the drive transmission changer utilizing an output shaft having an eccentric stud formed integrally with one end face thereof so as to extend parallel to, but laterally offset relative to the longitudinal axis of the output shaft.
This prior art drive transmission changer is operatively associated with a dog clutch comprised of a coupling shifter, engaged with a side face of the eccentric stud, and a counter-shift member drivingly coupled with the output shaft of the automotive power plant. Specifically, when the eccentric stud is rotated a predetermined angular distance in one direction about the longitudinal axis of the output shaft, the coupling shifter can be shifted linearly in a direction parallel to the axis of rotation of the counter-shift member until the coupling shifter is brought into engagement with the counter-shift member so that the rotation of the counter-shift member can be transmitted to the coupling shifter. On the other hand, when the eccentric stud is rotated in the reverse direction, the coupling shifter then engaged with the counter-shift member can be disengaged from the counter-shift member. In this way, the drive transmission changer is operable to selectively assume a drive coupling position to permit the automotive vehicle to be set in the four-wheel drive mode and a drive decoupling position to permit the automotive vehicle to be set in the two-wheel drive mode one at a time.
In order for the coupling shifter and the counter-shift member to be engaged with each other, the both must be held in phase matched relation with each other and, in order for the coupling shifter and the counter-shift member, which are engaged with each other, to be disengaged from each other, the engagement therebetween must not be firm and tough. However, if change of the drive transmission between the coupling shifter and the counter-shift member is forcibly performed, the drive motor used to drive the output shaft of the drive transmission changer will be loaded excessively. According to the Japanese publication referred to above, in order to avoid the excessive load from acting on the drive motor, a complicated control is effected to the drive motor to avoid the excessive load.
In contrast thereto, the Japanese Patent No. 3618112, first published Jul. 18, 1995 and patented Nov. 19, 2004, discloses the drive transmission changer of a structure, in which in the event that the drive transmission cannot be quickly changed in response to rotation of the drive motor, a driving force exerted by the drive motor is accumulated in a spiral spring with the drive motor then halted, but when the drive transmission is subsequently brought in position ready to change, the driving force so accumulated in the spiral spring is utilized to achieve the change of the drive transmission. Although the drive transmission changer disclosed in this Japanese patent is advantageous in that no complicated electric control of the drive motor is required, the drive transmission changer tends to become bulky because of the use of the spiral spring having a large maximum diameter.